The present invention relates to an extruder screw and a method for extruding polymers. More particularly, the present invention relates to an extruder screw and a method for extruding low melt viscosity liquid crystalline polymer.
Melt processable polymers which are capable of forming an anisotropic (crystalline) melt phase have been developed for use in making shaped plastic articles. These polymers have been found to be highly desirable to form shaped articles having improved mechanical properties. For instance, such shaped articles exhibit markedly smoother surface characteristics, are self-supporting and can be used at elevated temperatures for extended periods of time while being able to retain the desired mechanical characteristics at the elevated temperatures. Accordingly, these polymers are particularly useful for forming shaped articles including tubes, rods and molded articles.
Such polymers are described in greater detail, for example, in U.S. Pat. Nos. 3,991,013; 3,991,014; 4,066,620; 4,067,852; 4,075,262; 4,083,829; 4,093,595; 4,118,372; 4,130,545; 4,146,702; 4,153,779; 4,156,070; 4,159,365; 4,161,470; 4,169,933; 4,181,792; 4,183,895; 4,184,996; 4,188,476; 4,201,856; 4,219,461; 4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144; 4,238,598; 4,238,599; 4,238,600; 4,242,496; 4,245,082; 4,245,084; 4,247,514; 4,256,624; 4,265,802; 4,267,304; 4,269,965; 4,272,625; 4,279,803; 4,294,955; 4,299,756; 4,330,457; 4,318,841; 4,335,232; 4,337,190; 4,337,191; 4,339,375; 4,341,688; 4,347,349; 4,351,917; 4,351,918; 4,335,132; 4,355,134; 4,359,569; 4,360,658; 4,370,466; 4,374,228; 4,374,261; 4,375,530; 4,377,681; and 4,429,100, among others.
It is highly desirable to provide an extruder screw and method for permitting these polymers to be extruded rather than limiting their applications to injection molding and other processes. While various different extruder screw designs and configurations have been proposed in the past for different types of materials, there are no known extruder screws or methods particularly adapted for use with these liquid crystalline polymers.
For example, one extruder screw is disclosed in U.S. Pat. No. 4,000,884 issued to Chung. The Chung patent discloses an extruder screw including a feeding section connected to a melting section having a double helix design with a constantly decreasing solids channel profile and a constantly increasing melt channel profile. A transition section decreases the final melt channel depth continuously to a relatively small depth which is maintained constant in a metering section.
Another extruder screw is disclosed in U.S. Pat. No. 4,129,386 issued to Rauwendaal. The Rauwendaal patent discloses an extruder screw design including variations in both the helix angle and the depth of the channels. In particular, the Rauwendaal patent discloses a relatively constant depth channel in a feed section, a constantly decreasing channel depth (or flight height) in a transition section leading to a constant channel depth metering section. The Rauwendaal patent also discloses a double helix design in the transition section (FIG. 6). In addition, the Rauwendaal patent suggests an extruder screw speed of up to 120 rpm (Example 1, col. 5).
U.S. Pat. No. 4,321,229 issued to Blakeslee, III et al again discloses an extruder screw having a constant depth channel in the feeding section, and a constantly decreasing depth channel in the transition section. The final channel depth in the transition section is maintained in a metering section. The depth of the channel in the metering section is specified as being at least 0.25 inches.
Other patents disclosing extruder screws include U.S. Pat. No. 4,357,291 (Miller et al) which discloses an extruder screw having a feed section, a metering section and a second metering section. Different, substantially constant depth channels are provided within the respective sections. U.S. Pat. No. 4,173,445 (McKelvey et al) discloses a channel depth which constantly decreases from a feeding section to a metering or melt pumping section. Other extruder screw patents include Nos. 4,350,657 (James et al); 4,310,484 (Blakeslee III); and 4,280,802 (Lang et al).
It has been found that most known extruder screw designs are not acceptable for processing the low melt viscosity liquid crystalline polymers, particularly the liquid crystalline polyesters, due to the uniqueness of the material. That uniqueness is primarily directed to the rheology of the low melt viscosity liquid crystalline polymers.
It is the abnormally low viscosity of these polymers in the melted condition which causes problems in processing the material through known extruder screws. For example, the rheology of these particular polymers leads to a tendency toward backflow within the extruder screw which tendency is much greater than that encountered with other materials. Moreover, the flow of the liquid crystalline polymer is substantially different over the flights of the extruder screw than in the channels. The extremely low viscosity of the liquid crystalline polymers causes flow over the flights of the screw which flow is detrimental to the operation of the screw. Moreover, the low viscosity makes it extremely difficult to provide a sufficient driving force to the material to promote an even flow of material along the helix of the screw. The melted polymer, e.g., a polyester, tends to move in a more random pattern rather than smoothly along the channels of the screw. Due to these unique rheological characteristics, known extruder screws have been found to be inappropriate with these low melt viscosity liquid crystalline polymers. The problems associated with extrusion of such low melt viscosity liquid crystalline polymers is particularly acute with polyesters.
Accordingly, it is an object of the present invention to provide an extruder screw and a method of extrusion which are particularly adapted for use with melt processable polymers which are capable of forming an anisotropic melt phase. The extruder screw and method of the present invention are more particularly adapted for use with such polymers having a low melt viscosity and more particularly adapted for liquid crystalline polyesters.
It is a further object of the present invention to provide an extruder screw and method which produces a sufficiently high pressure for the extruded low viscosity material such that the material may be passed through a filter prior to formation into the desired shape. Still a further object of the present invention is to provide an extruder screw and method which are carefully controlled such that the problems associated with extrusion of the low melt viscosity liquid crystalline polymers are avoided.
Once again it is pointed out that due to the extremely low viscosity and the unique rheology of these polymers, the extruder screw and the extrusion method must be particularly adapted for use with these materials. It is the unique cooperation of the various portions of the screw, each of which must be carefully designed and controlled, that produces an acceptable material at the end of the extrusion process. That material must be uniformly melted and uniformly distributed. Moreover, the low viscosity material must be uniformly and efficiently moved along the extruder screw channel without substantial backflow or flow over the screw flights in order to produce acceptable extrusion pressure and constant flow to produce high quality products.
These objects and others are achieved by an extruder screw and extrusion method according to the present invention. The method of extruding the low melt viscosity polymers which form an anisotropic melt phase includes the steps of feeding the polymer to a feed section of an extruder screw. The feed is melted in a barrier section of the screw and the melted feed material is vented in a vent section. The material is then compressed over a comparatively low ratio in a compression section. The extruder screw is rotated at a high rate of speed to effect a high pressure discharge of material leaving the compression section.
In accordance with a further aspect of the method of the present invention, a large volume flow is produced through the compression section. In a preferred embodiment, the large volume flow is obtained by providing a relatively great depth initial channel in the compression section and also providing a relatively low compression ratio within that section while rotating the screw at a high rate of speed. It is essential that a high pressure output be obtained by the method in order that the material may be properly passed through a filter prior to formation. It is the combination of ensuring uniform melting and movement of the polymer in the barrier section and providing a relatively low compression ratio with a relatively large volume flow and rotating the screw at a high rate of speed which produces the high pressure and effective extrusion of the unique polymers.
In accordance with the present invention, the extruder screw includes a feed section and a barrier section which is preferably composed of a double helix with a constant depth melt channel and a decreasing depth solids channels adjacent to the melt channel. The flight between the melt and solids channels of the barrier section permits flow of the material from the solids channel to the melt channel. Moreover, by carefully controlling the transition from the barrier section to the vent section, the uniformity and continued movement of the melt is further enhanced. Each dimension of the extruder screw according to the present invention is carefully controlled since there is an extremely small "window" of the channel depth that can be successfully utilized to be effective due to the unique rheology of the low melt viscosity liquid crystalline polymer. Moreover, a driving arrangement is provided for rotating the screw of a high rate of speed.
In a preferred embodiment, the extruder screw has a ratio of compression which is 2.0:1 to 2.5:1, and most preferably about 2.5:1. Moreover, the extruder screw is rotated at a speed of 100 to 140 revolutions, most preferably 120 revolutions per minute. To further enhance the flow of the material through the screw, the surfaces of the screw are all chrome-plated. Still further, the dimensional tolerances of the screw are extremely small. For example, the concentricity of the root diameter of the channels to the outside flight diameter is within 0.005 inches. Further, the extruder screw and method according to the present invention produce a pressure of greater than 1000 pounds per square inch for the material at the end of the metering section.